Pressure and doping effects on the structural stability of thermoelectric BaAg2Te2

Ternary chalcogenides have attracted great attention for their potential applications in thermoelectric devices. Here, we investigate the pressure and doping effects on the structural stability of BaAg2Te2 using first-principles calculations. Imaginary frequencies are observed in the calculated phonon dispersions of the reported Pnma structure, indicating that Pnma BaAg2Te2 is lattice dynamically unstable at 0 K. Although the imaginary phonon frequencies are small, we find that hydrostatic pressure cannot effectively stabilize the structure. Based on the soft mode at Γ point, a new monoclinic phase with a space group of P21/c is proposed. From ab-initio molecular dynamics simulations, the P21/c phase is predicted to transform to the Pnma phase at a low temperature below 100 K. Electron/hole doping effects on the lattice dynamical stability of the Pnma phase are also studied. It is found that hole doping is superior to electron doping in stabilizing the Pnma phase. Further study on the electrical transport properties of the Pnma phase reveals a higher performance along b axis than that along the other two directions. This work paves an avenue to better understand the structural stability and electrical transport properties of thermoelectric BaAg2Te2.